The term split-cycle engine as used in the present application may not have yet received a fixed meaning commonly known to those skilled in the engine art. Accordingly, for purposes of clarity, the following definition is offered for the term split-cycle engine as may be applied to engines disclosed in the prior art and as referred to in the present application.
A split-cycle engine as referred to herein comprises:
a crankshaft rotatable about a crankshaft axis;
a power piston slidably received within a power cylinder and operatively connected to the crankshaft such that the power piston reciprocates through a power (or expansion) stroke and an exhaust stroke during a single rotation of the crankshaft;
a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft; and
a gas passage interconnecting the power and compression cylinders, the gas passage including an inlet valve and an outlet (or crossover) valve defining a pressure chamber therebetween.
U.S. Pat. Nos. 6,543,225, 6,609,371, and 6,952,923, all assigned to the assignee of the present invention, disclose examples of split-cycle internal combustion engines as herein defined. These patents contain an extensive list of United States and foreign patents and publications cited as background in the allowance of these patents. The term “split-cycle” has been used for these engines because they literally split the four strokes of a conventional pressure/volume Otto cycle (i.e., intake, compression, power and exhaust) over two dedicated cylinders: one cylinder dedicated to the high pressure compression stroke, and the other cylinder dedicated to the high pressure power stroke.
It is known in the art relating to aircraft engines to use radial engines for aeronautical applications. For example, radial engines were commonly used in World War II aircraft and in early model commercial airplanes. Radial engines are still presently used in some propeller-driven aircraft.
Radial engines differ from other common internal combustion engines such as inline and V-type engines in the arrangement of the engine cylinders. In a radial engine, the cylinders and corresponding pistons are arranged radially around the engine crankshaft in a circular pattern.
Radial engines are advantageous for airplane applications because they can produce a large amount of power, they have a relatively low maximum engine speed (rpm), avoiding the need for reduction gearing to drive propellers, and they are suitable for air cooling, eliminating the need for a water cooling system.
Although radial engines have been reliable aircraft engines and less expensive than other types of aircraft engines, use of radial engines in airplanes has substantially decreased. Conventional radial engines tend to be noisy and to consume more oil than other engine designs. Also, conventional radial engines have mechanical issues such as oil draining into the lower cylinders during non-use of the engine. This oil must be removed from the cylinders by turning the engine over by hand prior to starting the engine, which is an inconvenience to the pilot or the ground crew.
It is also known in the art of aircraft engines to use horizontally opposed engines, also known as “boxer” engines, to drive the aircraft's propellers. Boxer-type engines differ from other internal combustion engines in that the engine cylinders are arranged in a horizontally opposed relationship.
Horizontally opposed engines have the advantages of being more compact and having a lower center of gravity than other engine configurations. Horizontally opposed engines, like radial engines, potentially may be air-cooled, eliminating the need for a separate engine cooling system and thereby decreasing the overall weight of the engine. Therefore, horizontally opposed engines are suitable for aircraft applications. Horizontally opposed engines are also well balanced because each piston's momentum is counterbalanced by the corresponding movement of the piston opposing it. This reduces or may even eliminate the need for a balance shaft or counterweights on the crankshaft, further reducing the overall weight of the engine.
Horizontally opposed engines, however, are often noisier than other engine configurations such as V-type engines and inline engines. Also, horizontally opposed engines can be more difficult to fit into an engine compartment because horizontally opposed engines tend to be wider than other engine configurations.
It is further known in aeronautics that there are many uses in an aircraft for compressed air. However, conventional aircraft lack a convenient and efficient source of compressed air, thereby rendering these potential uses of compressed air infeasible.